1. Field of the Invention
The invention relates to a photoelectric conversion element suited for a thermophotovoltaic power generation device that converts radiant light emitted from a light-emitting body heated by a heat source into electric power by means of a photoelectric conversion element.
2. Description of the Related Art
As an art for directly obtaining electric energy from fossil fuel or combustible gas, power generation based on thermophotovoltaic energy conversion, namely, thermophotovoltaic power generation (TPV power generation) has been drawing attention. According to the mechanism of TPV power generation, radiant light is generated from a light-emitting body (radiant body or emitter) by applying combustion heat output from a heat source to the light-emitting body, and a photoelectric conversion element (solar cell) is irradiated with the radiant light so as to obtain electric energy. A TPV power generation device has no movable portions and thus can be realized as a noise-free and oscillation-free system. TPV power generation is advantageous in terms of cleanness, tranquility, and the like as a source of energy in the forthcoming generation.
For instance, Japanese Patent Laid-Open Application No. 63-316486 discloses a thermophotovoltaic power generation device composed of a light-emitting body made of a porous solid, light-emitting body heating means designed for the passage of exhaust gas through the light-emitting body, and a photoelectric conversion element for converting radiant energy radiated from the light-emitting body into electric energy.
In TPV power generation, infrared light obtained from a light-emitting body at a temperature of 1000 to 1700° C. is used. To convert light beams radiated from the light-emitting body and having a wavelength of 1.4 to 1.7 μm into electricity, it is necessary to use a photoelectric conversion element fabricated from a material having a small band gap (Eg). Because Si (silicon) as a generally employed material can convert only light beams having a wavelength equal to or smaller than 1.1 μm into electricity, it is not quite suited for a photoelectric conversion element.
A material having a band gap (Eg) of 0.5 to 0.7 ev is suited for a photoelectric conversion element designed for a TPV power generation device. Representative materials suited for a photoelectric conversion element include GaSb (gallium antimony, Eg=0.72 eV), InGaAs (indium gallium arsenic, Eg=0.60 eV), Ge (germanium, Eg=0.66 eV), and the like.
As a method of enhancing the energy efficiency of TPV power generation, reducing an amount of use of an expensive photoelectric conversion element, and thus reducing cost, the intensity of light beams generated by a light-emitting body is increased. If the intensity of light beams is multiplied by 100, the amount of use of the photoelectric conversion element is reduced to 1/100 thereof. As a result, the cost can be substantially reduced, and the energy conversion efficiency can be improved as well.
In this case, the amount of generation of electric current increases. Hence, the photoelectric conversion element in accordance with the related art is required to have a substantially increased area of a front surface side electrode so as to reduce a resistance loss. However, if the area of the front surface side electrode increases, the amount of light impinging on the photoelectric conversion element decreases, whereby a drawback, that is, unavailingness of an increased light intensity is caused.
On the other hand, there is a back-surface electrode type structure with no electrode on the front surface side. This structure is adopted in a beam-condensing power generation system. However, this back-surface electrode type is realized by using only an indirect transition type material having an increased carrier diffusion length. In fact, the back-surface electrode type is realized by using only Si. One of those indirect transition type materials having a small band gap is Ge (germanium). At the moment, none of photoelectric conversion elements made of Ge and adopting a back-surface electrode type as an electrode structure have been put into practical use.
Thus, in the specification and drawings attached to the application form of Japanese Patent Application No. 2000-105408, the applicant of this invention proposed a photoelectric conversion element whose element structure capable of substantially reducing a carrier recombination loss on a surface makes it possible to adopt Ge as a material suited for TPV power generation and to adopt a back surface electrode type as an electrode structure.
In the photoelectric conversion element proposed by the applicant, a diffusion layer for controlling movement of electrodes is provided as a means for reducing a carrier recombination loss on a light-receiving surface side. However, if a thermal diffusion method or an ion implantation method applied to a Si material is used as a method of forming the diffusion layer, the following problems are caused.
That is, according to the ion implantation method, ions at high energy levels are implanted in the surface of a substrate. Thus, the number of crystal defects on the surface increases, and the recombination loss increases. Hence, if the ion implantation method is applied to the light-receiving surface side where a large number of carriers are generated, there arises an obstacle to the enhancement of the photoelectric conversion efficiency.
Further, according to the thermal diffusion method, the diffusion layer is formed in an oxidative atmosphere. Thus, the Ge surface is inhomogeneously oxidized and becomes rough, whereby the number of crystal defects is increased. Accordingly, in this case as well, there arises an obstacle to the enhancement of the photoelectric conversion efficiency.